In a cellular communications network, user equipment (UE) (such as mobile telephones, mobile devices, mobile terminals, etc.) can communicate with other user equipment and/or remote servers via base stations. LTE systems include an Evolved Universal Terrestrial Radio Access Network (E-UTRAN) and an Evolved Packet Core (EPC) network (or simply ‘core network’). The E-UTRAN includes a number of base stations (‘eNBs’) for providing both user-plane (e.g. Packet Data Convergence Protocol (PDCP), Radio Link Control (RLC), Medium Access Control (MAC) and PHYsical (PHY) layers) and control-plane (e.g. Radio Resource Control (RRC)) protocol terminations towards the UE.
Depending on various criteria (such as the amount of data to be transmitted, radio technologies supported by the mobile telephones, expected quality of service, subscription settings, etc.), each base station is responsible for controlling the transmission timings, frequencies, transmission powers, modulations, etc. employed by the mobile telephones attached to the base station. In order to minimise disruption to the service and to maximise utilisation of the available bandwidth, the base stations continuously adjust their own transmission power and also that of the mobile telephones. Base stations also assign frequency bands and/or time slots to mobile telephones, and also select and enforce the appropriate transmission technology to be used between the base stations and the attached mobile telephones. By doing so, base stations also reduce or eliminate any harmful interference caused by mobile telephones to each other or to the base stations.
In order to optimise utilisation of their bandwidth, LTE base stations receive periodic signal measurement reports from each served mobile telephone (based on measurement configurations provided by the E-UTRAN), which include information about the perceived signal quality on a given frequency band used by (or being a candidate frequency band for) that mobile telephone. The served mobile telephones carry out measurements on reference signals that are transmitted at a known (non-zero) power level. By comparing the received power level to the reference power level, they are able to establish a measure of the signal degradation between the base station transmitting and the mobile telephone receiving the signal. On the other hand, interference is usually measured on resource blocks where transmissions of the serving base station are muted (i.e. set to zero power). That way, any signal that can be detected by the mobile telephone whilst the base station is known to transmit at zero power can be classified as interference caused by other transmitters operating in the same frequency band (e.g. neighbouring base stations and/or other mobile telephones). Depending on the measurement configurations (which are provided by the E-UTRAN), the mobile telephones generate and send measurement reports to their serving base stations. The measurement reports may be sent either periodically or when predefined events occur (e.g. interference gets higher than a predetermined threshold, signal quality falls below a predetermined level, etc).
These signal measurement reports are then used by the base stations in their decision to allocate certain parts of their bandwidth to the served mobile telephones and/or to adjust their transmission power and/or to hand over mobile telephones to other base stations (or other frequency bands/other radio access technologies (RATs)) when the signal quality does not meet the established criteria. The handing over of a mobile telephone might be necessary, for example, when the mobile telephone has moved away from the given base station, and also when a signal quality/interference problem has arisen.
A so-called Downlink Coordinated Multi-Point (CoMP) transmission/reception feature was introduced in Rel-11 of the 3GPP standards documentation to improve, for example, the coverage of high data rates for user equipment, temporary network deployment, cell edge throughput and/or to increase system throughput. The CoMP feature established techniques for compatible mobile telephones (and other user equipment) to communicate with multiple transmission points (TPs), substantially simultaneously. The TPs typically include (any combination of) base stations (eNBs), remote radio heads (RRHs), relay nodes (RNs), and the like. These techniques are described in, for example, TR 36.819 V11.1.0, the contents of which are hereby incorporated by reference. In summary, CoMP may be used i) to optimise received signal quality at the mobile telephone by transmitting the same signal from multiple TPs and/or ii) to increase data throughput by sending different signals (e.g. different parts of the user data) from different TPs concurrently (but of course without causing interference, e.g. by using different frequencies/timing/codes/etc).
When multiple transmission points are used by the mobile telephone, it is configured to measure and report the quality of the signals transmitted by each transmission point and also to measure and report back any interference experienced so that each transmission point can adjust its operation accordingly (i.e. to be able to transmit at/near an optimum power level and to keep interference to a minimum). Since the mobile telephone in this case is located within the overlapping coverage areas (cells) of multiple transmission points, these transmission points need to coordinate the transmissions of their reference signals, in order to make it possible to carry out the above described signal quality and interference measurements. In particular, when CoMP is used, the different transmission points transmit their respective reference signals at different times (whilst the other transmission points are muted), one by one, so that signal quality can be measured effectively by the mobile telephone, for each transmission point. Additionally, in order for the mobile telephone to able to measure interference caused by other transmitters than the cooperating transmission points, the base stations need to be muted, temporarily, at the same time, at least for the duration of the mobile telephone's measurements. Thus, the number of measurements (to be configured for and performed by the mobile telephone) equals to the number of transmission points (each one being a hypothetical interfering TP) plus one (for determining interference caused by other transmitters).
In Release-11, downlink CoMP has been specified to allow multiple transmission points (e.g. base stations) to coordinate their downlink data transmissions. In order to support more efficient utilisation of the downlink resources, the mobile telephone may be configured to report channel state information (CSI) by measuring a set of non-zero power (NZP) reference signal (RS or CSI-RS) resources—this set is known as the CoMP measurement set. For example, the mobile telephone may carry out measurement of a reference signal received power (RSRP) and report the results of this measurement to the base station which in turn can use the measurement to adjust its operation and to manage the CoMP measurement set (e.g. to choose a CoMP measurement set for which CSI feedback is required). The maximum size of the CoMP measurement set is three NZP CSI-RS resources, selected from all possible CSI-RS resources (defined as a CoMP Resource Management Set).
The mobile telephone may also be configured to perform one or more interference measurements (CSI-IM). Each CSI-IM is associated with one interference measurement resource (IMR), which is a set of resource elements on which interference measurements can be made.
In a so-called ‘CSI process’, the E-UTRAN can request the mobile telephone to carry out a combined measurement on a NZP CSI-RS resource and on an IMR. The mobile telephone performs the combined measurements on resources indicated by the ‘CSI process’, and sends a so-called ‘CSI report’ to the E-UTRAN, which includes the results of the combined measurements. The mobile telephone can be configured to perform, in response to a given CSI process, periodic and/or aperiodic CSI reporting.
A new LTE transmission mode (‘Transmission mode 10’ or ‘TM10’) has also been defined in Rel-12 to provide support for CoMP functionalities. The relevant parameters of transmission mode 10 are defined in 3GPP TS 36.213 (v11.1.0), the contents of which are incorporated herein by reference. In particular, section 7.1 of TS 36.213 describes scrambling identities for UE-specific reference signal generation, supported DCI formats and transmission schemes. Section 7.2 describes that a mobile telephone in transmission mode 10 can be configured with one or more CSI processes per serving cell (by higher layers). Each CSI process is associated with a CSI-RS resource (defined in Section 7.2.5) and a CSI-interference measurement (CSI-IM) resource (defined in Section 7.2.6). A CSI reported by the mobile telephone corresponds to a CSI process configured by higher layers. Each CSI process can be configured with or without PMI/RI reporting by higher layer signalling.
In Rel-12, in order to enhance small cell performance, mechanisms for interference avoidance and coordination between macro and small cells as well as among small cells are currently being considered. However, since clusters of relatively small cells are typically denser than scenarios considered for the so-called Enhanced Inter-Cell Interference Coordination (enhanced ICIC or eICIC) technique in Rel-10, or for the so-called Further Enhanced ICIC (FeICIC) technique and CoMP in Rel-11, these techniques cannot be re-used without added complexity to the user equipment and/or transmission points.
Furthermore, the carrier aggregation (CA) feature defined for LTE-Advanced supports transmission bandwidths up to 100 MHz of spectrum by aggregating the resources of two or more component carriers. When carrier aggregation is used there are a number of serving cells, one for each component carrier. The radio resources connection is handled by one cell, the primary serving cell, served by the primary component carrier (PCC), whilst user data may be communicated via any of the component carriers, primary and/or any secondary component carrier (SCC). However, the effective coverage of and/or perceived signal qualities offered by the various serving cells may differ—either due to the different frequencies used in different cells or due to power planning considerations (and possibly other factors influencing propagation of transmitted signals). Therefore, the base station configures the mobile telephones it is serving via its component carriers to carry out and report predetermined signal quality and interference measurements (i.e. one or more CSI processes, depending on the number of cells to be measured) so that it can take appropriate corrective actions when signal degradation is experienced by user equipment within its cell(s).
3GPP has recently made a working assumption (at RANI meeting #71) that for the joint operation of downlink CoMP and CA, the UE capability for the number of supported CSI processes is defined as follows:                PCSI is the maximum number of CSI processes supported on a component carrier;        PCSI is provided per band combination;        The PCSI value applies to each component carrier within a band; and        PCSI can take a value in {1,3,4}.        
In this context, band combination refers to a collection of bands. Therefore, it can be seen that for a mobile telephone which is capable of performing up to a maximum of four simultaneous CSI processes in transmission mode 10 (for both single carrier operation and carrier aggregation), and assuming that there are five bands (component carriers) aggregated, this means that there is always at least one band in which the mobile telephone cannot process any CSI processes.